[FEAT] terms table: basic cache for the search and pagination filters (now they're restored after page reload)

[FIX] prevent nodes_ngrams_count.items() KeyError by forcing nodes_ngrams_count to use same key as ngrams_data when key is truncated

- mail notification improved
- import url from settings
- message upated (links to gogs removed in the message, still present in
  the interface)

- Graphs are in an ordered list
- Informations with start/end and number of nodes
- User can choose to compute with conditional or distributional distance

[FIX] BUG ISI Parser (which depends on RIS parser)
      Adding a space in the regex breaks ISI parser
      Then the question is: is the space really usefull for RIS parser ?

unittests: revert the use of setUpClass for test node creation: the idea was valid but it triggers bug as in code.djangoproject.com/ticket/23325

unittests: mini_test_samples for parsers: pubmed, europresse, istex and ris (zotero variant) working now with tests_090_toolchain

risparser: correct ris tags comparing to specification in 'https://en.wikipedia.org/wiki/RIS_(file_format)'